The present invention generally relates to open magnets used in magnetic resonance imaging (MRI) systems and, more particularly, to such an open magnet with recessed field shaping coils.
As described in U.S. Pat. No. 5,999,075 to Laskaris et al. assigned to the assignee of the present invention, an open magnet of a MRI system typically includes two spaced-apart coil assemblies which are substantially mirror images of one another. The space between the coil assemblies contains an imaging volume and allows for positioning of a patient in that volume and access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. Each coil assembly typically has an annular-shaped main coil with a longitudinal axis, an annular-shaped shielding coil coaxially aligned along the longitudinal axis and spaced longitudinally outward and apart from the main coil, and a cylindrical-shaped magnetizable pole piece disposed about the longitudinal axis between the main and shielding coils.
The magnetizable pole pieces of the coil assemblies enhance the strength of the magnetic field produced by the main coils. Further, by selectively configuring the inner surfaces of the pole pieces, the open magnet is magnetically shimmed so as to improve homogeneity of the magnetic field. The current practice is to form several annular steps on the inner surfaces of the pole pieces which protrude to different heights or distances into the space between the coil assemblies in order to control the magnetization distributions of the pole pieces and thus shape the magnetic field to create the homogeneous field volume in the space between the coil assemblies for MRI imaging. However, such magnetic field shaping annular steps occupy some of the space between the coil assemblies thus reducing the imaging volume of the MRI system and complicating the magnet cryogenic structure design. Moreover, the field shaping capacity of the step technique is limited and reduced as the main field increases since more load areas of the stepped pole pieces become magnetically saturated.
Accordingly, it is desirable to provide an open magnet which provides a highly uniform, high field, open field of view for MRI with a well-contained stray field.
An actively shielded open magnet useful for MRI applications comprises magnetized pole pieces, superconducting main and bucking coils, and recessed field shaping coils to provide a highly homogeneous, high field, open field of view for MR imaging with a wellcontained stray field.
The magnetized pole pieces comprise a ferromagnetic material and have superconducting field-shaping coils situated in annular grooves formed in the pole face. The magnetized pole pieces with annular recessed field-shaping coils are placed inside a cryogenic helium vessel of the magnet. The uniform cryogenic temperature inside the cryogenic vessel avoids the field fluctuation that would otherwise result from temperature changes of the pole pieces. A pair of superconducting main coils generates the high magnet field in the imaging volume. Unlike many of the open MRI magnets with warm iron poles, the cold magnetized pole pieces do not form a return path for the magnetic flux. Instead, the stray field is contained by the superconducting bucking coils.
The recessed field shaping coils advantageously result in more usable space between the two coil assemblies of the open magnet, enable higher field and simplify the manufacturing of the pole pieces and the magnet cryostat structure.